Diel means "daily" in the sense of a 24-hour period rather than the time between sunrise and sunset. Thus a "diel variation" is a variation that occurs regularly once per 24-hour period.
In many sources, the word diurnal is used in the same sense. However, in more modern or more technical usage, diurnal is reserved for the meaning "during the daytime", i.e. in contrast with nocturnal (during the nighttime) or crepuscular (during the twilight). "Diel" is therefore to be preferred where its use would not be pedantic.
Notable examples of diel phenomena from biology include:
- circadian rhythm, the "internal body clock" that regulates the 24 hour cycle of biological processes in animals and plants
- daily vertical migration of some forms of plankton which can move several hundreds of meters each day
